The present invention relates to an improved apparatus and method for the drawing of glass fibers and is particularly concerned with such an apparatus wherein the orifice plate of the drawing bushing is of the type having a generally planar undersurface toward which bulk flow gas is directed to achieve fiber cooling and attenuation. The invention is especially directed to an improved orifice pattern which provides for "self-healing" in the event of the breakage of a fiber being drawn from the plate.
In its more specific aspects, the invention is concerned with an improvement in the apparatus disclosed in U.S. Pat. No. 3,905,790, granted Sept. 16, 1975, by Edward T. Strickland. That application discloses a method and apparatus for forming glass fibers wherein the orifice plate has a generally planar undersurface and bulk flow gas is directed upwardly toward the undersurface to effect fiber cooling and attenuation. It also suggests that self-correction of localized flooding can be achieved by close orifice spacing and discloses a technique of such self-correction wherein capillary grooves are provided between the orifices to provide a path for controlled glass flow from one orifice to another in the event of breakage of the fiber emanating from one of the orifices.
When glass defects (e.g., stones, crystalline particles, cords and seeds) pass through conventional tipped bushings, they generally cause fiber breakage. Then, the loose tail of the broken fiber either snaps out the rest of the fibers being drawn from the bushing, or else the drop that drains from the tip grows until it falls and breaks the other fibers. Either result causes an interruption of the fiber forming process.
When similar defects pass through a non-tip bushing using a column of rapidly moving cooling gas to maintain fiber separation, the fiber is similarly broken, but does not cause a snap-out of the other fibers. The drop left behind grows until it meets a cone of glass supplying a fiber being drawn from an adjacent orifice. It then, at times, causes a break of the fiber being drawn from the cone, which in turn floods to the next adjacent fiber, creating a "domino" effect that requires the operator's immediate attention.
In the preferred form of the non-tip bushing disclosed in aforementioned U.S. Patent, the inventor contemplates the provision of capillary grooves between the orifices in order to provide for controlled flooding in the event that a fiber breaks. The capillary grooves are designed to cause the plate to act as though it had controlled, but perfect, wetability. Since only a small volume of glass from the oozing orifice will first contact the neighbor fiber, the increase of acceleration load on the neighbor fiber will be gradual and, as the whole fiber pulls more glass out of the groove, the fiber cross-section enlarges and the fiber becomes stronger until a single larger fiber is fed by two orifices. Although the capillary grooves are effective in that they encourage more rapid flooding to selective adjacent orifices, they have some disadvantages. For example: they reduce the strength of the orifice plate; they affect the flow of electrical current, thus producing hot and cold spots; and, they increase plate fabrication costs.